Matthew Targett is likely to come in for Bertrand at left-back on Sunday, when Saints skipper Fonte is likely to return whether the manager keeps with the 3-5-2 formation or not.
Shane Long, Dusan Tadic and Victor Wanyama could come in after starting from the bench in the Boxing Day win at Crystal Palace, but Jay Rodriguez (cruciate ligament), Sam Gallagher (knee) and Jack Cork (knock) remain out.
Chelsea manager Jose Mourinho has a full squad to choose from.
The Blues boss has no injury concerns or suspended players, but is likely to make changes for the Blues’ second match in three days.
Andre Schurrle, John Obi Mikel, Filipe Luis and Didier Drogba are among those who could come in.